Cathode-ray tube signal comparator



Sept. 6, 1949. D. E. suNsTEiN CATHODE-RAY TUBE SIGNAL CQMPARATOR Filed Feb. 261, 1947 Patented Sept. 6, 1949 CATHODE -RAY TUBE SIGNAL COMPARATEUR David E. Sunstein, Cynwyd, Pa., assignor to Philco Corporation, Philadelphia, Pa., a corporation of Pennsylvania.

Application February 26, 1947, Serial No. 730,885

The invention herein described and claimed relates to improvements in cathode ray tube structures. More specifically it relates to a particular arrangement of an electrostatic deflecting electrode within a cathode ray tube, whereby the tube is rendered especially suitable for use in a comparator of the waveforms of electrical signals occurring in time-spaced relationship. Such a comparator is particularly useful in radar target motion indicating systems.

Electrical signal waveform comparators are well known in which the electron beam of a cathode ray tube is caused successively to scan the tube screen under the control of signals occurring in time-spaced relationship whose'waveforms are to be compared. If an electron beam is caused repeatedly tov describe the same trace on a screen consisting of adielectric material, such as glass, a point is reached, after but a, few scans, at which no change in secondary electron emission from the screen obtains between successive scans. However, when a new trace is scanned, differing at least in part from the old one, a change in secondary emission results. If secondary electrons emitted by the screen are drawn away to a collecting electrode as rapidly as they are emitted by the screen, this scanning of a new trace will produce a change in the net charge on the screen. This change is readily'detectable by means of a capacitive pick-up electrode'disposed in proximity to the screen, and which may be substantially coextensive therewith.

In an arrangement oi this sort it is convenient to increase the effective'length of the trace, and hence the duration of signals which can be compared, by causing the electron beam to describe a spiral trace on the screen and by modulating the deflection of the beam radially to cause ,it to depart from said trace in response to signals which-are to be compared. Circuits are known for producing the required damped sinusoidal deecting waves for application to the horizontal and vertical deflecting plates of a, conventional electrostatically deflected cathode ray tube to produce such a spiral trace. In the past, however, some diiculty has been experienced in providing for the radial deflection of the cathode ray beam in the course of describing the spiral trace. One method of accomplishing this is to modulate the normal horizontal and vertical delecting signals, used to produce the spiral trace, by the input signal. This, however, necessitates the .use of separate balanced modulatorsone for each deflecting signal.

These must either be of f 6 Claims. (Cl. Z50- 158) z tubescreen, in which undesired ratherlarge power output or, alternatively, additional amplication must be provided, which introduces further din'culties because of the wide frequency band which must be passed. It was recognized that radial deiiection from the spiral path might conveniently be produced by means of an electrode axially disposed with reference to the tube axis. This, however, would either requi-re a lead from this electrode to be taken off through the side of the tube envelope in such manner that it would interfere with the electron beam so as to produce discontinuities in the trace; or, alternatively, itwould require Ya lead from the axial electrode through the screen itself at a point corresponding approximatelyto the center of the spiral trace. VThis latter method has not until now been considered feasible owing to the presence of the capacitive output electrode in the vicinity of the screen which, by capacity coupling. would tend to pick up signals from the lead in addition to those induced in it from the screen itself.- Attempts to counteract the effect of such coupling by neutralization have not proven successful because of the large ratio of the undesired to the desired signal. l

Accordingly the principal objects of theV invention are:

(l) To provide an improved form of cathode ray tube particularly adapted for use in an electrical signal waveform comparator; and

(2) To provide an improved cathode ray tube structure of the sort above referred to, comprising an axially disposed radial deecting electrode and a capacitive electrode associated with the coupling between said deecting and said capacitive lelectrodes is minimized.V l Y According to the invention, means are provided wherebythe lead from the axial electrode is carried out through the screen and through an aperture in the capacitive output electrode, while at the same time undesirableV coupling between the lead and the output electrode is effectively avoided. To this end the lead is1electrostatically shielded from the output electrode. The shield employed may be of tubular form, sealed into the screen of the cathode ray Vtube and extending appreciably within .the tube envelope in the direction of the electron gun or beam-forming means. It is preferably constructed to have sucient rigidity so that it need be supported only atthe point where it is sealed into the tube screen. Within it, and insulated from it, is disposed an electrode which protrudes within the tube envelope apprecie-bly beyond the end of the shield so as to provide an active beam-deflecting portion. At some point within the shield there is provided a substantially airtight seal between the shield and the lead. Further means may be provided within the shield-for example, insulating sleeves of ceramic or other suitable materialto 'insure that' the lead' willl remain substantially coaxially disposed withfrelation to the shield.

The structural details of an embodiment of the invention will be more fully undcrstpodand other objects and features thereoi""'will" become apparent from a consideration of the following description with reference to accompanying.

drawings, in which:

Figure 1 illustrates a' cathode ray tube, in accordance with the invention, connectedin asignal comparator circuit, and

Figure 2 illustrates, in detail, the structure of the axial deecting electrode and associated shield 'as' employed in thev tube illustrated'in Figure 1.

"Referring now to Figure 1, the embodiment thereillustrated, of a cathode ray tube in accordance with" the invention, comprisesan evacuated nvelp'eof' Vglass 'or other' suitable dielectric Iriateri'al"` customarily employed for this purpose. This envelope maybe of the usual conguration, o'fn'prising' 'auf' elongated relatively oonstrioted ncl portion and a' substantially frusto-conicalA portion." The smaller endof't'he latter is joined' tofthe'neck-'portion`, While'tlielarger end is closed' by 'a substan'ti'all'y plane' porti'omforming a screen upon' which 'electroiis'are adapted to impingel.

The other'eiid'ofthe'oonstriotedaneok portion may be' 'ofovideuwith a' baser; of/Bakeiite or Vother Suitable 'material having contac'tpins' embedded therein toi provide connections" frornrthe yexterior ir-'the' tute tot'iie various 1elements"-witioin it, as is 'customary in' cathode ra'y' devices. VWithin the cori's'tricte'd neck portion, inthe "vicinity of b'a'fseZ; are provided'm'eans for generating, form'- I, ad'con'trolling'an' electron'b'eam' which may normally be" directed "along" the axis." or'th'e tube so as to impinge on screen@ These means may s comprise the' 'usual cathode, intensity 'controlling grid and anodes employed for this purpose, and' here 'generally represented byoyiinoer `3. Spaced'axially from' this electrongunbr beam# forming means" are disposed horizontal and vertical' electrostatic dee'cting" plates!! and 5 =for theV prpos'eff d'ee'cting 'the "electronbeam norizontaiiy and vertically as' iii conventional athd'ray tu'b'e's. Likewise, as in the usuall form or eatiiode'ray'tube, the interior of thefrusto! cni'c'alp'ortion ofthe' tube'envelope may befalmost completely covered with an aquad'ag 'coating 8. When the tube is in operation, thiscoating may be grounded, 'through' av suitable' connection IU, so'as'to' prevent'the accumulation of electrical chargesv on the interior surface of the glass,'i'n' re ponse to Secondary emission from lscreen I9 on tf en the team impinges. Thuis' undesiratie random'd'ee'ction of 'the electron beam is avoided. -rIn accordance `with the invention," 'a tubular conductive shield I2 is seaiedinto'the screen 9 of th't'ube envelpev'so as to enteliit Substantially centrally yand extend an 4appreciable distance within' the't'ube envelope in a` direction toward ele'ctron'gun 3 and substantially'coinciding with the 'axis ofth'e tube. s shown vin the drawing this' shield 'may extend within the tube appreciably b'ey'ond the' aq'uadag coating 8.' Within siiieirlji; ami insulated therefrom, is disposed a conductive electrode I3 which eXtends'Ioe-'yon'd' the shield at both ends. Its exposed portion within the tube envelope provides a deiiecting electrode adapted to produce radial deflection of the cathode ray beam in response to deflecting signals supp-lied to the portion of the electrode which protrudes from the other end of the shield.

Aiso there may be provided' on the interior surface of'the envelope :in the vicinity of the portion of electrode I3 which is exposed, a second coating of aquadag which is distinct and insulated from the coating 8. This coating provides a-i'rusto-conical annular conductive surface which is adapted to cooperate with the interior exposedpo'rtion of electrode I3 to produce radial deiie'ction ofthe cathode ray beam. Further, in

' raccordance withY the invention, the exterior of the screcn portieri 9 of the tube envelope is covered with a conductive coating forming a capacitive' pick-up or output electrode, through the medium of which it is possible to detect changes in the electrical charge produced on the screen portion in response to bombardment by. the electron beam and the resultant emission. of secondary electrons therefrom. This coating is insulated from shield I2 by leavingy a relatively. narrow uncoated ring completely surrounding the shield. The coating may comprise a thin lm of'gol'd; which may be of the order of onehalf wavelengthin thickness for light,..`and hence transparent. Alternatively, it is possible to use a relatively fine wire mesh screen. cemented to the face of the tube. Although it is not necessary to the operation of theY tube in accordance with the invention that screen- 9: be provided with an internal coating'o phosphor, sov as to' produce a visible tracev in response to bombardment by the electron beam; it. may beconvenient to include.

Such a. coating to be used for test purposes.

As shown i'rrr Figure 2, shield I2, of the tube according to Figure 1, may comprise a relatively short tubular' portion: i4: andv a relatively longer tubular' portion I5.' .The latter is preferably formed in one piece and comprises. longitudinally contiguous portions of .progressively diminishing crossxse'ctionf. .This 'is don-e in order to provide adequate rigidity of the structure, While'at the same time reducing its cross section in the por.

tion which is nearest tothe source of the electron beam.".Thus interference'by the shield with the beam is minimized. Preferably both shield I2 and electrode I3 are of chrome-iron having a coecient ofV expansion closely approximating that of glass. By constructing the shield in two pieces, the manufacture of the tube is simplie'd, in Vthat the tubular portion I4 may rst be Sealed in thescreen portion'of the envelope', and later thelon'ger tubular portion I5 canbesoldered Within the shorter tubular portion I4. Also there maybe provided' a collary I1 which is adapted to `reinforce 'the' junction between 'the' tubular portions It and I andimprove the quality of the'vacuum seal at this point'. Electrode I3 is disposed within the tubular shield, anda glass seal' 'I9 -is'provided between 'it andthe shield, preferably at the most'constricted portion of tneiatt'er. "To center theeieotroue within'the shield'and to insulate it therefrom, there may b e' provided 'tubular cerarni'ccollars 20 and 2l disposed'at'suitable intervals within th'e shield. "In operating the tube 'in accordance ywith the invention',j dampedsinusoidal"oscillations in substantially phase-'quadrature relationship are sup-Y piiefiomouroe's 24 endet, through eoniie'etioris plates' 4 and"'5. V"'I'hese' Signals arel adapted' to pro- 5 duce deflection of the cathode ray beam so as to cause it to describe a trace of substantially spiral form on screen 9 of the tube. To the outer end of electrode I3 are supplied video input signals which may be derived from a conventional radar equipment (not shown). These signals will. produce radial deflection of the electron beam and will cause it to depart from the normally spiral trace produced in response to the signals applied to deilecting electrodes li and 5 from sources 24 and 25. The amount of such departure at a given point on the trace will, of course, depend on the instantaneous magnitude of the video input signal to electrode I3. The coating II on the inner surface of tube envelope I may be grounded as shown, or, if preferred, the video input signals may be applied in push-pull relationship to electrode I3 and coating I I. In either case the electrodeand the coating will cooperate to produce deflection of the beam. If these signals recur at the same rate. as the spiral trace on the tube screen, the cathode ray beam will be deflected by like amounts at corresponding points on successive traces. Hence successive traces will be identical, and an equilibrium condition will soon be established (usually as soon as a single trace has been scanned) in which n0 change in secondm ary emission from the screen takes place between successive scans. This will be the case when the signal from the radar is produced in response to signals from fixed targets only. However, if the input signal to electrode I3 fails to recur at the trace repetition rate, the beam will not be delected from the spiral trace by like amounts at corresponding points on successive scans, and successive traces will differ. Thus the beam will be caused to describe a new area of the screen and the secondary emission from the screen will change. This will be the case when the signals from the radar are produced in response to tar- L gets in motion, and hence do not recur periodically. The change in secondary emission will Aproduce a like change in the charge on screen 9, which, in turn, will induce a signal in the conductive coating deposited on the external surface of screen 9. This signal may be supplied through connection 26 to a suitable video amplier 2l to yield an output at terminals 2B and 29 which will be indicative of departures of the video input signals from periodicity at the trace repetition rate.

ture of the tube, in accordance with the invention, is susceptible of considerable variation from that shown and described, without departing from the scope of the invention as defined by the appended claims.

. claim:

1. A cathode ray tube structure comprising: means for forming an electron beam; a screen upon which said beam is adapted to impinge to cause secondary electrons to be emitted, said screen being displaced from said beam forming means and extending transversely to the path of said beam; a deflecting electrode disposed interjacent said beam forming means and said screen and substantially displaced from said screen; a connection for applying deflecting signals to said deflecting electrode, said connection extending toward and passing through said screen; a conductive electrode at least partially coextensive with 4 said screen and adapted capacitively to respond Such departures may, in a system of this sort, be v taken as indicative of target motion.

Thus, by the invention, there is provided a cathode ray device in which the electron beam can be deflected so as normally to describe a i spiral or circular trace on the tube screen; in which, in response to signals applied to an axially disposed deilecting electrode which enters the tube envelope through the tube screen, the beam is adapted conveniently to be deflected radially to cause departures from said trace; in which, by means of a conductive coating on the external surface of the tube screen in the vicinity of said electrode and overlying at least a portion of said screen, there are conveniently derived signals which vary in response to variations in the secondary emission from said screen produced by alterations in the trace scanned by said beam; and in which, by means of a conductive shield enclosing said axial electrode throughout an appreciable portion of its length, said conductive coating is prevented from responding capacitively to the deflecting signals applied to the axially disposed deflecting electrode.

It will, of course, be understood that the struct-o variations in the electrical charge on said screen owing to variations in the secondary emission from said screen; and a conductive shield interposed between said last-named electrode and said connection for appreciably reducing capacitive coupling between said output electrode and said connection.

2. A cathode ray tube structure comprising: means for forming an electron beam; a screen upon which said beam is adapted to impinge to cause' secondary electrons to be emitted, said screen being displaced fromlsaid beam forming means and extending transversely to the path of said beam; a conductive electrode disposed in proximity to said screen and adapted capacitively to respond to variations in the electrical charge on said screen owing to variations in the secondary emission from said screen; a deflecting electrode disposed interjacent said beam forming means and said screen and substantially displaced from said screen; a connection for applying deflecting signals to said deecting electrode, said connection passing in the vicinity of said output electrode; and a conductive shield surrounding at least a portion of said connection for appreciably reducing capacitive coupling between said first-named electrode and said connection.

3. A cathode ray tube structure comprising: an envelope of dielectric material, means enclosed in said envelope for forming a beam of electrons and for directing said beam against a portion of said envelope to cause it to emit secondary electrons, a tubular conductive shield sealed into said envelope and extending appreciably within said envelope in the direction of said beam forming means, an electrode disposed within said shield and insulated therefrom, said electrode extending within said envelope appreciably beyond said shield to provide a beam deecting portion, and a conductive electrode at least partially coextensive with said portion of said envelope, said lastnamed electrode being electrically insulated from said shield and adapted capacitively to respond to variations in electrical charge on said portion of said envelope owing to Variations in the secondary emission from said screen.

4. A cathode ray tube structure comprising: an envelope of dielectric material, means disposed within said envelope for forming an electron beam adapted to impinge upon the inner surface of said envelope to cause it to emit Secondary electrons, an electrode extending through the wall of said envelope and an appreciable distance within said envelope in the direction of said beam 75 forming means, a conductive shield enclosing said 7. electrode throughout its length except for a portion displaced from said envelope wally in the direction of said beam forming means, said eX- posed portion being adapted to eiTect deflection of said beam in response to potentials applied to said electrode, and a capacitive electrode disposed in proximity to a portion of the Wall ci said envelope immediately surrounding said firstnamedelectrode and said shield.

5. A cathode ray tube structure comprising: an envelope of dielectric material, means disn posed within said envelope for forming an electron beam adapted to impinge upon the inner surface of said envelope to cause it to emit second-- ary electrons, an electrode extending through the Wall of said envelope and an appreciable distance within said envelope in the direction of said beam forming means, a conductive shield enn closing said electrode throughout its length eX- cept for a portion ldisplaced from said envelope wall in the directionof said beam formingrmeans, an annular electrode substantially coaxial with said first-named electrode and substantially coextensive axially with the unshielded portion of said first-named electrode, said annular electrode being adapted to cooperate with the unshielded portion of said first-named electrode to eiect de- :Flection of said electron beam in response to deecting signalsy applied to at least one of said electrodes, and a capacitive electrode disposed in proximity to a portion of the wall of said envelope immediately surrounding said first-named electrode and said shield.

6. A cathode ray signal detector.. comprising means for forming an electron beam; a screen upon which said beam is adapted to imping to,

cause secondary electronsY to be vemitted; said screen being displacedfroin said beam f'cain'iingY means and extending transversely to the path of said beamj a delecting electrode disposed interjacent said beam forming means and said screen,

reducing appreciably capacitive coupling .between said conductive means and said connection.

DAVID E. sUNsT'EiN;

REFERENCES CITED The -following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,929,067 Hund Oct. 3, 19334 2,200,745 Heymann May 14, 1940 2,328,259 Christaldi et a1. Aug. 31, 1943 2,408,415 Donaldson Oct. 1, 1946 2,412,965 Chevigny et al. Dec. 24, 1946 

